Differential signaling is a method of transmitting information with two complementary signals on two conductors, such as paired wires. Differential signaling usually improves resistance to electromagnetic interference (EMI) since the information is conveyed through the difference between the voltages on the wires. However, if there are imbalances or asymmetries between the two conductors, common mode components may arise even when the two conductors are differentially driven. The presence of common mode currents on a cable does not inherently degrade the integrity of differential signaling, but if energy can be transferred from a common mode to a differential mode, then the common mode current can become a dominant interference signal, in a phenomenon known as mode-conversion or mode coupling.
Mode conversion can cause significant performance degradation. While internal interference sources (such as ISI, echo, FEXT, and NEXT) are known to the link partners and can be cancelled effectively with cancellers and equalizers, the mode conversion interference is unknown until it occurs, and thus presents difficulties for the desired performance of high bandwidth communication systems.